1. Field of the Invention
The present invention relates to a transconductor with consideration given to linearity of input-output characteristics.
2. Description of the Related Art
A transconductor having MOS transistors as an input differential pair generally does not have linearity in input-output characteristics, and requires the approach for obtaining linearity. For example, in the technique disclosed in Zhenhua Wang and Walter Guggenbuhl: “A Voltage-Controllable Linear MOS Transconductor Using Bias Offset Technique”, IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 25, NO. 1, Feb. 1990, the second differential pair in which a differential-input voltage is inputted by being level-shifted is provided in addition to the first differential pair in which the differential-input voltage is inputted, and both of them are connected so that polarities from the differential voltage input to differential current outputs of them are opposite from each other, whereby linearity is enhanced. This configuration cancels out the nonlinearity which the first differential pair has by the nonlinearity which the second differential pair has, but has the circuit which performs level shift, and increases power consumption correspondingly.
Meanwhile, a planar type is the main stream of the structure of the present MOS transistors, and in the planar type, the channel is controlled by one gate. With microfabrication of transistors, a leak current between drains and sources has become a problem, and the leak current is a current leaking through a substrate even when the voltage between the gate and source is sufficiently lower than the threshold voltage. This prevents power saving. The leak current is caused by the phenomenon called punch through in the short channel effect.
On the other hand, it is known that by adopting the structure capable of controlling a channel with a plurality of gates, the effect of suppressing punch through can be obtained. The transistors having two gates are called Dual-gate FinFETs. Of such transistors, the one capable of individually controlling two gates is disclosed in “CMOS Vertical Multiple Independent Gate Field Effect Transistor (MIGFET)”; Mathew, L. et. al.; SOI Conference, 2004; Proceedings. 2004 IEEE International; 4-7 Oct. 2004 Page(s): 187-189. This document further describes that the threshold value voltage can be made variable with the second gate.